The present invention relates to a device and a method for molding coils placed around core teeth of a rotor core of an electric motor with resin material.
Conventionally, it is known that when the coil wire for motor coils is subjected to a tension as it is passed through core slots of a rotor core, the edges of the core teeth could damage the insulating film of the coil wire. Therefore, it has been customary to design a coil forming machine for forming motor coils around core teeth in such a manner as to avoid applying tension to the coil wire. Once the coils are placed around the core teeth, the rotor core is covered by insulating material such as thermosetting resin.
In such a motor production process, typically, the rotor core is preheated after the coils are placed in the core slots, and the resin material is then coated over the rotor core. The resin coating is adjusted, and is cured. After the resin coating has fully cured, the outer profile of the rotor is cut into a prescribed shape by machining. Although the resin coating process can be carried out by using an automated machine, the process of adjusting the resin coating is required to be carried out manually. Therefore, a significant time period (for instance about 2 hours) must elapse from the start of the pre-heating process to the completion of the curing process. Therefore, the coil wire which has been received in the core slots may spring back out of the core slot during this time period, and a projecting part of the coil wire could be cut by the machine tool during the process of shaping the motor rotor.
This can be avoided by using wedges to prevent the spring back of the coil wire as illustrated in FIG. 5. After the coils 8 are wound around the core 4 of the rotor 3, and before the resin coating is formed, a wedge 11 is inserted into each core slot 4a from an axial end of the rotor core 4 in such a manner as to be engaged by a laterally expanded part of each core tooth 4b, and prevent the wire of the coils 8 from expanding radially out of the core slot 4a. However, when the rotor 3 has a large number of core slots 4a, a corresponding number of wedges 11 have to be inserted in the core slots 4a, and this leads to an increase in the material cost and a complication of the production process.
Also, according to the conventional motor rotor, the shape of the coil wire extending from the axial end of one core slot into another or the shape of the coil end is relatively indeterminate, and it has been necessary to leave a significant amount of resin coating deposited in this part because of the fear of cutting into the coil wire when cutting the resin coating away from this part. The excessive deposition of resin coating is not desirable as it increases the moment of inertia of the motor rotor, and therefore reduces the performance of the motor.
U.S. Pat. No. 5,634,258 issued Jun. 3, 1997 discloses a method for molding motor coils in resin material which uses the flow of the resin material to control the spring back of the coil wire. The contents of this patent are hereby incorporated in this application by reference.
In view of such problems of the prior art, a primary object of the present invention is to provide a method and a device for molding coils placed around a motor core with resin material which can reduce both the material cost and the production cost.
A second object of the present invention is to provide a method and a device for molding coils placed around a motor core with resin material which can improve the performance of the motor.
According to the present invention, these and other objects can be accomplished by providing a motor coil molding device for molding coils wound around a motor rotor with resin material in a die assembly, the die assembly including upper and lower die halves which can be closed so as to define a cavity for receiving a motor rotor coaxially therein, and opened so as to allow placing and removing the motor rotor in and out of the cavity, the cavity including: a main part for receiving a rotor core coaxially therein so as to define a small annular gap between the rotor core and the opposing die surface; support portions for supporting two shaft ends of the motor rotor, typically, in a sealing relationship; and intermediate parts each defined between the main part and a different one of the support portions; at least one of the intermediate parts including a small diameter portion defining a substantially smaller inner diameter than the main part.
This device can be conveniently used for carrying out a motor coil molding method comprising the steps of: placing a motor rotor in the die cavity; radially constraining a coil end at an axial end of the rotor core with coil end constraining means, such as a small diameter portion defining a substantially smaller inner diameter than the main part in an intermediate part defined between the main part and one of the support portions; injecting resin material from a radially peripheral part of the die assembly so as to force coil wire received in core slots radially inward by a flow of the resin material; and curing the resin material deposited around the motor rotor. Typically, the method further comprises the step of removing the resin material from an outer circumferential surface of the motor rotor by machining.
Thus, because the coil end of the motor rotor can be appropriately covered by the smaller diameter portion during the molding process, the deposition of the resin material over the coil end can be minimized so that the costs for the material and the production can be reduced, and the moment of inertia of the motor rotor can be reduced. Because the coil wire at the coil end is pushed radially inward, the moment of inertia of the rotor owing to the mass of the coil wire is reduced for this reason also. To effectively retain the coil wire in each core slot against the tendency of the coil wire to spring back, the die assembly may include an injection hole for introducing the resin material from a radially peripheral part of the die assembly. The flow of the resin material is thereby directed radially inward, and then axially in either direction in the die cavity so that the flow of the resin material counteracts the tendency of the coil wire to spring back.
The small diameter portion may either extend from an axial end portion of the cavity to a region adjacent an axial end of the rotor core or be defined by an annular ridge having a relatively small axial width. The small diameter portions may be formed by two halves formed in the upper and lower die halves in either case.